Many automobiles use diesel engines, particularly, large-size vehicles. In recent years, there is a strong demand to reduce the discharge of PM, as well as nitrogen oxide, carbon monoxide, hydrocarbon, etc., from the exhaust gas of a diesel engine. Therefore, not only development of a technique for reducing PM, for example, by improving engines or optimizing combustion conditions, but also establishing a technique for efficiently removing PM from exhaust gas, has been desired.
In order to remove PM from exhaust gas, a ceramic honeycomb filter, alloy filter and ceramic fiber filter are generally used. However, when PM is trapped in such a filter, oxidative removal of PM cannot be expected to be achieved by only the heat of the exhaust, resulting in the filter being clogged with trapped PM as operation time passes, giving rise to an increase in air-flow resistance, which places a load on the engine.
To solve this problem, regarding the oxidative removal of PM trapped in a filter, Japanese Unexamined Patent Publication (Kokai) No. 2001-280121 discloses a technique where a filter loaded thereon with a PM oxidation catalyst is used, an electric heating device is disposed upstream of this filter and when the oxidation of PM by the PM oxidation catalyst cannot be efficiently performed, the exhaust gas is heated by an electric heating device. This patent publication also discloses disposing an electrical discharge device upstream of the filter, and if desired, causing an electrical discharge to generate an oxidative gas such as nitrogen dioxide (NO2).
Incidentally, in order to trap PM, it is known to electrically charge PM and promote PM trapping by using an electrostatic action. For example, Kokai No. 2004-19534 discloses a PM purification apparatus having an electrical discharge electrode and a honeycomb-shaped counter-electrode. When this PM purification apparatus is used, an electrical discharge is generated between these electrodes thereby electrically charging the PM and causing PM to accumulate on the honeycomb-shaped counter-electrode. This patent publication also discloses loading a PM oxidation catalyst on the honeycomb-shaped counter-electrode and promoting the oxidation of PM by a catalyst.
Furthermore, in order to promote trapping of PM by an electrostatic action, Kokai No. 2004-239257 discloses a PM purification apparatus where PM in an exhaust gas is electrically charged freely in advance and at the same time, an electric field that is non-parallel to the exhaust gas flow is created in a honeycomb support, thereby promoting the electrostatic PM trapping in the honeycomb support. This patent publication also discloses that a ceria (CeO2)-based PM oxidation catalyst is trapped in the honeycomb support to promote the oxidation of PM by using the catalyst, and the air-flow resistance in the outer periphery-side of the honeycomb support is made larger than the air-flow resistance in the inner-side region of the honeycomb support.
In addition, in Kokai No. 2005-12098, there is in a problem that the flow rate of an exhaust gas passing through the center part of the filter is larger than the flow rate of an exhaust gas passing through the outer periphery side thereof and accordingly, the filter is readily clogged due to accumulation of PM in the center part of the filter. In order to solve such a problem, this patent publication discloses an exhaust gas purification apparatus where by trapping PM by an electrostatic action, a honeycomb support with a cell density that is gradually decreased from the central part toward the outer periphery side is used so that the flow rate of exhaust gas in the outer periphery side of the honeycomb support can be increased and PM can be uniformly accumulated in the entire honeycomb support. This patent publication also discloses loading a ceria (CeO2)-based PM oxidation catalyst in the honeycomb support and promoting the oxidation of PM by using the catalyst.
In these PM purification apparatuses, good PM purification performance is achieved by using an electrostatic action for the PM trapping or by using a PM oxidation catalyst for the oxidative removal of PM. Accordingly, the present invention provides an apparatus and a method for PM purification, where these PM purification apparatuses are further improved.